Arrays of light emitting diodes (“LEDs”) are utilized for a wide variety of applications, including for general lighting and multicolored lighting. Because emitted light intensity is proportional to the average current through an LED (or through a plurality of LEDs connected in series), adjusting the average current through the LED(s) is one typical method of regulating the intensity or the color of the illumination source. Solid state lighting, such as LEDs, are typically coupled to a converter as a power source.
A step-down (Buck) converter can be controlled either in discontinuous conduction mode (DCM) or continuous conduction mode (CCM). Typically DCM is suitable only for low power processing, while CCM mode is utilized for higher power conversion, such as for high brightness LEDs.
In the prior art, a technique referred to as “current programming mode” (“CPM”) is utilized in an attempt to simplify compensator designs for the Buck converter, see, e.g., U.S. Pat. Nos. 6,034,517; 4,975,820; 4,672,518; 4,674,020; and 4,717,994. Prior art circuits for this CPM mode typically regulate the inductor current in CCM mode around a set point within the Buck converter. This set point is further manipulated by an outer compensating loop. For a Buck converter implemented with CPM mode, the outer compensating loop can be a single pole network.
A CPM implementation, however, cannot simply utilize a controller for a Buck converter, but must also be accompanied with a circuit implementing DCM. One challenge facing this CCM implementation is that the control system needs to transition between DCM and CCM modes in both directions. Many prior art control systems will oscillate around these two modes, which causes LED current to fluctuate, and which may be visually apparent as flicker, for example. When the outer compensator bandwidth may be low, another problem with this CCM technique is that the LED current may also fluctuate, particularly when the input voltage to the Buck converter contains a high ripple percentage.
Most prior art LED control systems also utilize a “high side sensing” technique, in which the output current of a Buck converter is sensed by a sensing resistor in series with the inductor (see, e.g., U.S. Pat. Nos. 6,853,174; 6,166,528; and 5,600,234). With high side sensing, output current can be regulated accurately, and high side sensing can also be utilized with CPM techniques. In order to overcome the various stability problems and other disadvantages mentioned above, the prior art has utilized various controllers to implement hysteretic (or so called “bang-bang”) control to regulate this inductor current.
The high side sensing technique works well when the controller integrated circuit (“IC”) can tolerate the Buck converter input voltage range. This is typically not the case for an LED driver, however, which involves input voltages which are much higher than what a controller IC is capable of tolerating or specified to tolerate and, accordingly, such a high side sensing technique cannot be utilized with typical controller ICs.
Various techniques for “low side sensing” are also found in the prior art, in which the sense resistor is in between the main converter switching element (MOSFET) and ground, see, e.g., U.S. Pat. Nos. 6,580,258 and 5,912,552. The low side sensing technique is usually associated with a control method called “constant off time” (U.S. Pat. Nos. 6,580,258 and 5,912,552). Detailed analysis of this constant off time method shows that while it may be suitable for controlling Buck converter output voltage, it exhibits a very large error if it is used for controlling output current, due to converter component and environmental variations (e.g., manufacturing variations, component aging or life span, and environmental conditions such as temperature).
A need remains, therefore, for a control method, apparatus and system, using low side sensing and suitable for IC implementation, that can regulate output current accurately while eliminating those drawbacks caused by existing techniques. Such an apparatus, method and system should provide a simpler controller compared to CPM techniques, and further provide excellent accuracy and not exhibiting the problems associated with prior art techniques such as CPM technique. Such an apparatus, system and method should also control the intensity (brightness) of light emissions for solid state devices such as LEDs, while simultaneously providing for substantial stability of perceived color emission and control over wavelength shifting, over both a range of intensities and a range of LED junction temperatures. Such an apparatus, system and method should be capable of being implemented with few components, and without requiring extensive feedback systems.